Oil and gas wells are conventionally drilled with drill pipe to a certain depth, then casing is run and cemented in the well. The operator may then drill the well to a greater depth with drill pipe and cement another string of casing. In this type of system, each string of casing extends to the surface wellhead assembly.
In some well completions, an operator may install a liner rather than another string of casing. The liner is made up of joints of pipe in the same manner as casing. Also, the liner is normally cemented into the well. However, the liner does not extend back to the wellhead assembly at the surface. Instead, it is secured by a liner hanger to the last string of casing just above the lower end portion of the casing. The operator may later install a tieback string of casing that extends from the wellhead downward into engagement with the liner hanger assembly.
When installing a liner, the operator drills the well to the desired depth, retrieves the drill string, then assembles and lowers the liner into the well. During assembly, a cement shoe with a check valve will normally be secured to the lower end portion of the liner as the liner is made up. When the desired length of liner is reached, the operator attaches a liner hanger to the upper end portion of the liner, and attaches a running tool to the liner hanger. A liner top packer may also be incorporated with the liner hanger. The operator then runs the liner into the wellbore on a string of drill pipe attached to the running tool. The operator sets the liner hanger and pumps cement through the drill pipe, down the liner and back up an annulus surrounding the liner (i.e., between the wellbore wall and the liner). The cement shoe prevents backflow of cement back into the liner. The operator then sets the liner top packer, if used, releases the running tool from the liner hanger and retrieves the drill pipe.
A variety of designs exist for liner hangers. Some may be set in response to mechanical movement or manipulation of the drill pipe, including rotation. Others may be set by dropping a ball or dart into the drill string, then applying fluid pressure to the interior of the string after the ball or dart lands on a seat in the running tool. The running tool may be attached to the liner hanger or body of the running tool by threads, shear elements, or by a hydraulically actuated arrangement.
In another method of installing a liner, the operator runs the liner while simultaneously drilling the wellbore. A drill bit is disposed at the lower end portion of the liner. This method is similar to a related technology known as casing drilling. One option is to not retrieve the drill bit, rather cement it in place with the liner. If the well is to be drilled deeper, the drill bit would have to be a drillable type. This technique is disfavored when components are desired to be retrieved from downhole. Such components may include one or more of downhole steering tools, measuring while drilling instruments and retrievable drill bits. Retrievable bottom hole assemblies are known for casing drilling, but in casing drilling, the upper end portion of the casing is at the rig floor. In typical liner drilling, the upper end portion of the liner is deep within the well and the liner is suspended on a string of drill pipe. In casing drilling, the bottom hole assembly can be retrieved and rerun by wireline, drill pipe, or by pumping the bottom hole assembly down and back up. With liner drilling, the drill pipe that suspends the liner is much smaller in diameter than the liner and has no room for a bottom hole assembly to be retrieved through it. If unable to retrieve and rerun the bottom hole assembly, the operator would not be able to liner drill with expensive directional steering tools, logging instruments and the like, without planning for removing the entire liner string to retrieve the tools.
Some liner drilling proposals instruct coupling a bottom hole assembly to a string of drill pipe and running the drill pipe to the bottom of the liner. Retrieving the drill string at the conclusion of the drilling would retrieve the bottom hole assembly.
Alternatively, if the operator wishes to retrieve the bottom hole assembly before cementing the liner, or if the operator wishes to rerun the bottom hole assembly and continue drilling with the liner, a process as disclosed in U.S. Patent Application Publication 20100282463 may be used. As disclosed therein, an operator may install a profile sub at a bottom portion of a liner string being made up and latch a bottom hole assembly to the profile sub. The bottom hole assembly includes the drill bit. The operator secures a liner hanger assembly to an upper portion of the liner string. Once the liner string is made up, the operator secures a lower end portion of a drill pipe string to an upper portion of the liner string and lowers the liner string on the drill pipe string until the drill bit reaches a bottom of the well. The drill bit may then be rotated to drill the well deeper. At a selected depth, the operator releases the bottom hole assembly from the profile sub and reverse circulates drilling fluid through a liner annulus surrounding the liner string. The reverse circulation pumps the bottom hole assembly up the liner string. When the bottom hole assembly reaches the lower end portion of the drill pipe string, the operator latches the bottom hole assembly to a catcher tool disposed at the lower end portion of the drill pipe string. The operator moves the liner hanger assembly into setting engagement with the casing string to support the weight of the liner string. Then the drill pipe string may be released from the liner string and retrieved along with the bottom hole assembly. If not at total depth, the operator may re-run the drill pipe string and the bottom hole assembly back into the casing string and re-securing the lower end portion of the drill pipe string to the upper portion of the liner string. The operator releases the liner hanger assembly from setting engagement with the casing string and releases the bottom hole assembly from the lower end of the drill pipe string. The operator pumps fluid down the drill pipe string to move the bottom hole assembly down the liner string latch into the profile sub. Drilling may then continue.
In casing drilling, the bottom hole assembly is spaced out at the shoe using a casing drilling drill lock assembly (DLA). The bottom hole assembly is relatively short, and spacing-out the under reamer arms with respect to the shoe ensures the system will drill new hole. This task is easily performed by measuring the short distance (approximately 20-30 feet) between the under reamer and the DLA, and the short distance (approximately 20 feet) from the casing shoe to the latching profile sub of the DLA. The difference in these distances indicates the location of the bottom hole assembly with respect to the shoe. These distances may be adjusted to facilitate the bottom hole assembly's ultimate location to ensure efficient drilling.
In contrast, during liner drilling, the bottom hole assembly is coupled to the liner near the top of the liner. The liner length varies depending on the job, and the liner top may be 1000 to 5000 feet or more away from the bottom hole assembly. To perform the under reamer space-out task per the steps discussed in casing drilling, the field engineer would need to have extremely accurate liner and drill pipe tallies, as there is little to no room for error in calculating this distance. Additionally, there is no secondary indication on the accuracy of the space-out. If the space-out is not done properly as the system is assembled on the rig floor (such as by mis-measurement or miscounted joints), the error can be undetected until after the liner is at the bottom of the hole, resulting in a mis-run. The potential cost on a deepwater project would be in the millions of dollars.
A method proposed to properly locate the bottom hole assembly includes use of a Hall-effect or magnetic-type sensor. The system pulses a signal to the surface once the bottom hole assembly is properly landed so that the floor personnel can continue to make-up the system and/or drill. Unfortunately, the space-out of this type of system needs to be accurate to within a few inches to ensure proper engagement of the reaming device or needs a thrusting sub to space-out the bottom hole assembly with respect to the liner shoe. In some instances, debris and other issues with the system have failed to produce a signal to surface, causing a mis-run with the system. Such a system is also somewhat complicated and expensive.